Sinus, are the hollow spaces within the skull bone. It is because of these hollow spaces that we do not feel the actual weight of the brain and skull in entirety. These hollow spaces secrete mucus that keep the nose (nostrils) moist. This moisture forms the protective layer. A healthy sinus is otherwise hollow filled with air.
Sinusitis (sinus+itis) is the inflammation of these hollow spaces which results in excessive mucus production that fills within the sinus and blocks the air circulation.